Diffusion barriers for metallic superconducting wires

ABSTRACT

In various embodiments, superconducting wires incorporate diffusion barriers composed of Nb alloys or Nb—Ta alloys that resist internal diffusion and provide superior mechanical strength to the wires.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/639,530, filed Mar. 7, 2018, the entiredisclosure of which is hereby incorporated herein by reference. Thisapplication is also a continuation-in-part of U.S. patent applicationSer. No. 15/964,723, filed Apr. 27, 2018, which is a continuation ofU.S. patent application Ser. No. 15/696,330, filed Sep. 6, 2017, whichclaims the benefit of and priority to U.S. Provisional PatentApplication No. 62/383,676, filed Sep. 6, 2016, the entire disclosure ofeach of which is hereby incorporated herein by reference.

TECHNICAL FIELD

In various embodiments, the present invention relates to the formationand processing of superconducting wires that incorporate diffusionbarriers for prevention of interdiffusion.

BACKGROUND

A superconducting material exhibits no electrical resistance when cooledbelow its characteristic critical temperature. Although high-temperaturesuperconductor materials, which have critical temperatures higher thanthe 77K boiling point of nitrogen, have been identified, these materialsare often exotic (e.g., perovskite ceramics), difficult to process, andunsuitable for high-field applications. Thus, for practicalsuperconducting applications requiring wires and coils and bundlesthereof, the metallic superconductors Nb—Ti and Nb₃Sn are most oftenutilized. While these materials have critical temperatures below 77K,the relative ease of processing these materials (e.g., drawing intowires), as well as their ability to operate at high currents and highmagnetic fields, have resulted in their widespread use.

Typical metallic superconducting wires feature multiple strands (or“filaments”) of the superconducting phase embedded within a copper (Cu)conductive matrix. While Nb—Ti is sufficiently ductile to be drawn downinto thin wires directly, its applicability is typically limited toapplications featuring magnetic fields having strengths belowapproximately 8 Tesla. Nb₃Sn is a brittle intermetallic phase thatcannot withstand wire-drawing deformation, and thus it is typicallyformed after wire drawing via diffusion heat treatment. Nb₃Snsuperconducting materials may typically be used in applicationsfeaturing magnetic fields having strengths up to at least 20 Tesla.Thus, several different techniques have been utilized to fabricateNb₃Sn-based superconducting wires. For example, in the “bronze process,”a large composite is fabricated from Nb rods and Cu—Sn alloy rods (thatinclude, e.g., 13-15% Sn) surrounding the Nb rods. Since these materialsare ductile, the composite may be drawn down to a suitable diameter, andthen the drawn-down composite is annealed. The heat treatment results ininterdiffusion and the formation of the Nb₃Sn phase at the interfacebetween the Nb and the Cu—Sn. Other processes for forming Nb₃Sn-basedsuperconducting wires similarly involve formation of the brittle Nb₃Snphase after wire drawing. For example, pure Sn or Sn alloys with Cu orMg may be incorporated in the interior of the initial composite andannealed after drawing. Alternatively, Nb filaments may be embeddedwithin a Cu matrix and drawn down into wire. The resulting wire maysubsequently be coated with Sn. The coated wire is heated, forming aSn—Cu phase that eventually reacts with the Nb filaments to form theNb₃Sn phase.

While the techniques detailed above have resulted in the successfulfabrication of metallic superconducting wires utilized for a host ofdifferent applications, the resulting wires often exhibit insufficientelectrical performance. Typical superconducting wires contain many ofthe Nb₃Sn or Nb—Ti filaments described above embedded within, disposedaround, and/or surrounded by a Cu or Cu-containing stabilizer thatprovides the wires with sufficient ductility for handling andincorporation within industrial systems. Although this stabilizer is notitself superconducting, the high electrical conductivity of Cu canenable satisfactory overall electrical performance of the wire.Unfortunately, various elements from the superconducting filaments(e.g., Sn) may react with portions of the Cu-based stabilizer, forminglow-conductivity phases that negatively impact the overall conductivityof the entire wire. While diffusion barriers have been utilized toshield the stabilizer from the superconducting filaments, these barrierstend to have non-uniform cross-sectional areas and may even locallyrupture due to non-uniform deformation during co-processing of thediffusion barrier and the stabilizer. While such diffusion barrierscould simply be made thicker, such solutions impact the overallconductivity of the wire due to the lower electrical conductivity of thediffusion barrier material itself. For example, for cutting-edge andfuture applications such as new particle accelerators and colliders,magnets are being designed beyond existing wire capabilities; such wireswill require a non-copper critical current density of more than 2000A/mm² at 15 Tesla. As the diffusion barrier is part of the non-copperfraction, minimizing the volume of any barrier material is importantwhile any strength benefit is advantageous.

In view of the foregoing, there is a need for improved diffusionbarriers for metallic superconducting wires that substantially preventdeleterious reactions involving a stabilizer or various elements (e.g.,Cu) while remaining uniformly thin so as not to occupy a significantamount of the overall cross-sectional area of the wire.

SUMMARY

In accordance with various embodiments of the present invention, asuperconducting wire and/or precursor thereof (e.g., a compositefilament utilized to form the wire) features one or more diffusionbarriers including, consisting essentially of, or consisting of aniobium (Nb) alloy. A diffusion barrier is typically disposed between atleast a portion of the Cu wire matrix and the superconducting filaments,and/or between the superconducting filaments and a stabilizing elementincorporated within and/or around the superconducting wire foradditional mechanical strength. In accordance with embodiments of theinvention, monofilaments may each include, consist essentially of, orconsist of a Nb-based core within a Cu-based (e.g., Cu or bronze(Cu—Sn)) matrix, and stacked assemblies of the monofilaments may bedisposed within a Cu-based matrix and drawn down to form compositefilaments. Thus, composite filaments may each include, consistessentially of, or consist of multiple Nb-based monofilaments within aCu-based matrix. A diffusion barrier in accordance with embodiments ofthe invention may be disposed around each composite filament when thecomposite filaments are stacked to form the final wire, and/or adiffusion barrier may be disposed around the stack of compositefilaments and between the stack of composite filaments and an outer Custabilizer or matrix.

In various embodiments, composite filaments are disposed within aCu-based matrix (e.g., a Cu-based tube) and drawn down into thesuperconducting wire (or precursor thereof) and heat treated. One ormore of the composite filaments may themselves incorporate a diffusionbarrier therein, and/or a diffusion barrier may be disposed within theCu-based matrix of the superconducting wire and around the compositefilaments. In various embodiments, the diffusion barrier includes,consists essentially of, or consists of a Nb—W alloy including, e.g.,0.1% to 20% W, 0.2% to 15% W, 0.2% to 12% W, 0.2% to 10% W, 0.2% to 8%W, or 0.2% to 5% W. For example, the diffusion barrier may include,consist essentially of, or consist of an alloy of Nb and approximately11%-12% W (i.e., Nb-12W) or an alloy of Nb and approximately 5%-6% W(i.e., Nb-6W) or an alloy of Nb and approximately 2.5%-3% W (i.e.,Nb-3W). In various embodiments, the diffusion barrier includes, consistsessentially of, or consists of a Nb—W alloy (e.g., Nb-12W, Nb-6W, orNb-3W) with one or more additional alloying elements therein, e.g.,alloying elements such as Ru, Pt, Pd, Rh, Os, Ir, Mo, Re, and/or Si.Such alloying elements may be present in the diffusion barrierindividually or collectively at concentrations up to 5% by weight oreven up to 10% by weight (e.g., between 0.05% and 10%, between 0.05% and5%, between 0.1% and 3%, between 0.2% and 2%, between 0.2% and 1%, orbetween 0.2% and 0.5%). In various embodiments of the invention, weldsformed of Nb—W alloys incorporating one or more of these additionalalloying elements may have grain structures that are more equiaxedtoward the center of such welds; thus, welded tubes formed of thesematerials for use as diffusion barriers may exhibit superior mechanicalproperties and processability when drawn down to small sizes during wirefabrication.

In accordance with various embodiments of the invention, a diffusionbarrier may include one or more alloying elements such as W, Ru, Pt, Pd,Rh, Os, Ir, Mo, Re, and/or Si. Such alloying elements may be present inthe diffusion barrier individually or collectively at concentrations upto 5% by weight or even up to 10% by weight (e.g., between 0.05% and10%, between 0.05% and 5%, between 0.1% and 3%, between 0.2% and 2%,between 0.2% and 1%, or between 0.2% and 0.5%). In various embodiments,filaments and/or diffusion barriers in accordance with embodiments ofthe invention may be substantially free of Mg, B, Fe, Al, and/or Ni.

In various embodiments of the invention, a diffusion barrier mayinclude, consist essentially of, or consist of an alloy or mixturecontaining both Nb and tantalum (Ta), as well as one or more alloyingelements such as W. For example, the diffusion barrier may include,consist essentially of, or consist of an alloy of Nb, Ta, andapproximately 2.5-3 atomic % W (i.e., Nb—Ta-3W), with or without one ormore of the alloying elements listed above. In various embodiments,diffusion barriers including, consisting essentially of, or consistingof Nb—Ta—W alloys may contain W in concentrations of, e.g., 0.2-12atomic %. Diffusion barriers in accordance with embodiments of theinvention may include at least 1% Ta, at least 5% Ta, at least 8% Ta, atleast 10% Ta, at least 15% Ta, at least 20% Ta, at least 25% Ta, atleast 30% Ta, at least 35% Ta, at least 40% Ta, or at least 45% Ta.Diffusion barriers in accordance with embodiments of the invention mayinclude at most 50% Ta, at most 45% Ta, at most 40% Ta, at most 35% Ta,at most 30% Ta, at most 25% Ta, at most 20% Ta, at most 15% Ta, at most10% Ta, at most 5% Ta, or at most 2% Ta.

Diffusion barriers in accordance with embodiments of the invention mayinclude, consist essentially of, or consist of an alloy or mixturecontaining Nb (or Nb and Ta) and one or more alloying elements insteadof (or in addition to) W. For example, such alloying elements mayinclude C and/or N. References herein to diffusion-barrier alloyscontaining W are understood to encompass alloys containing alloyingelements such as C and/or N instead of or in addition to W.

Nb-alloy diffusion barriers in accordance with embodiments of theinvention may also exhibit advantageous ductility due at least in partto low oxygen contents and/or high levels of purity. For example,diffusion barriers in accordance with embodiments of the invention haveoxygen contents less than 500 ppm, less than 200 ppm, less than 100 ppm,or even less than 50 ppm. The oxygen content may be at least 0.5 ppm, atleast 1 ppm, at least 2 ppm, or at least 5 ppm. In addition oralternatively, diffusion barriers in accordance with embodiments of theinvention may have purities exceeding 99.9%, or even exceeding 99.99%.

Advantageously, Nb-alloy diffusion barriers in accordance withembodiments of the invention have refined grain structures (e.g., smallaverage grain sizes) when compared to conventional diffusion barriermaterials, and this enables the deformation and processing of thediffusion barriers within the superconducting wire to be substantiallyuniform without localized thinning that can rupture the diffusionbarrier and compromise the performance of the wire. The small grain sizeof the diffusion barriers (e.g., less than 20 μm, less than 10 μm, lessthan 5 μm, between 1 and 20 μm, or between 5 and 15 μm) results from thepresence of the alloying element(s), and thus diffusion barriers inaccordance with embodiments of the invention need no additionalprocessing (e.g., forging such as tri-axial forging, heat treatments,etc.) to produce the refined grain structure. Thus, overallmanufacturing costs and complexity may be reduced via use of diffusionbarriers in accordance with the present invention.

The superior grain structure and/or mechanical properties of diffusionbarriers in accordance with embodiments of the present invention enablethe diffusion barriers to provide protection from deleterious diffusionwithin the superconductor wire without occupying excessive amounts ofthe cross-sectional (i.e., current-carrying) area of the wire. (Incontrast, the use of various other diffusion barriers with lessermechanical properties and/or less refined grain structures would requirethe use of larger barriers that would deleteriously impact theductility, conductivity, and/or various other properties of the finalwire.) Wires in accordance with embodiments of the present inventionexhibit little or no interdiffusion with the Cu matrix while retaininggood high-field, high-current superconducting properties below theircritical temperatures.

The use of Nb-alloy diffusion barriers advantageously enables less ofthe cross-section of the superconducting wire to be occupied by thediffusion barrier, and thus more of the cross-section may be occupied bycurrent-carrying superconducting filaments. However, diffusion-barriermaterials in accordance with embodiments of the invention alsoadvantageously provide additional mechanical strength to thesuperconducting wire while retaining good high-field, high-currentsuperconducting properties below their critical temperatures. In variousembodiments, the mechanical strength of wires may facilitate mechanicaldeformation of the wire (e.g., winding, coiling, etc.) withoutcompromising the electrical performance of the wire and/or withoutcausing cracks or fractures in, or otherwise compromising the mechanicalstability of, the wire and/or its filaments. In various embodiments, thediffusion barrier(s) may collectively or individually occupy at least0.5%, at least 1%, at least 2%, at least 3%, at least 4%, at least 5%,or at least 7% of the cross-sectional area of the final wire. In variousembodiments, the diffusion barrier(s) may collectively or individuallyoccupy less than 20%, less than 15%, less than 12%, less than 10%, lessthan 9%, less than 8%, less than 7%, less than 6%, less than 5%, lessthan 4%, less than 3%, or less than 2% of the cross-sectional area ofthe final wire. In this manner, the diffusion barrier(s) within thesuperconducting wire provide, in accordance with various embodiments,the wire with a minimum yield strength (e.g., after any heat treatmentof the wire and/or of the filaments) of at least 75 MPa, at least 100MPa, or even at least 150 MPa. Instead or in addition, wires containingone or more diffusion barriers in accordance with various embodimentsexhibit ultimate tensile strengths of at least 250 MPa, at least 300MPa, or even at least 350 MPa. In various embodiments, the diffusionbarrier(s) may collectively or individually occupy more than 25% of thecross-sectional area of the final wire, and/or less than 35%, or lessthan 30% of the cross-sectional area of the final wire. Mechanicalproperties such as yield strengths and ultimate tensile strengths ofwires in accordance with embodiments of the present invention may bemeasured in accordance with ASTM E8/E8M-15a, Standard Test Methods forTension Testing of Metallic Materials, ASTM International, WestConshohocken, Pa., 2015, the entire disclosure of which is incorporatedby reference herein.

The enhanced mechanical strength of superconducting wires in accordancewith embodiments of the invention advantageously enables such wires towithstand Lorentz forces exerted on the wire during operation at highmagnetic field strengths. As known in the art, the “self-field” in amagnet winding is higher than the center-line field and is highest atthe innermost winding. In addition, the current required to create thefield is the same in all the wire in the magnet. The Lorentz force isF=B×I (i.e., magnetic field crossed into current), and the generatedfield is directly proportional to the current I; thus, the force isproportional to the square of the current. For example, the Lorentzforces will be four times higher at 16 Tesla compared to 8 Tesla. Thus,as the applied magnetic field is increased in magnitude, the mechanicalstrength of the wire to withstand the force (perpendicular to bothcurrent and field, via the cross-product relation) must be higher aswell. Superconducting wires in accordance with embodiments of theinvention may be advantageously deployed for applications utilizingmagnetic fields having strengths of at least 2 Tesla, at least 5 Tesla,at least 8 Tesla, or even at least 10 Tesla, i.e., magnetic fluxdensities of at least 20,000 gauss, at least 50,000 gauss, at least80,000 gauss, or even at least 100,000 gauss.

In addition, since diffusion barriers in accordance with embodiments ofthe invention include Nb, in various embodiments a portion of thediffusion barrier may advantageously react (e.g., with Sn or Ti) duringthe wire fabrication process (e.g., during one or more heattreatment/annealing steps) to form a superconducting phase (e.g., Nb₃Snor Nb—Ti) that contributes to superconductive conductivity of the finalwire. In such embodiments, the thickness of the diffusion barrier istypically sufficiently large to prevent the entire diffusion barrier (orat least the Nb therewithin) from reacting, and thus the remainingunreacted portion of the diffusion barrier provides not only resistanceto interdiffusion but also increased mechanical strength (due to, e.g.,the presence of alloying elements such as W). In various embodiments,the reacted portion of the diffusion barrier is therefore present withinthe wire as an annular (or other shape, mimicking the shape of thediffusion barrier) reacted region. In various embodiments, non-Nballoying elements (e.g., Ta, W, etc.) may not react to form thesuperconducting phase in the reacted region, and thus those elements maybe expelled from the reacting portion of the diffusion barrier duringthe reaction. Thus, at the interface between the reacted portion of thediffusion barrier and the unreacted, remaining portion of the diffusionbarrier, one or more (or even all) of such non-Nb elements may bepresent at a concentration higher than that within the portion of thediffusion barrier opposite the reacted portion. In other embodiments,the unreacted, remaining portion of the diffusion barrier contains ahigher concentration of one or more such non-Nb elements than waspresent prior to the reaction (e.g., when the diffusion barrier wasintroduced during the wire fabrication process). Thus, even though aftera portion of the diffusion barrier reacts to form a superconductingphase, the thickness of the remaining diffusion barrier is reduced, thehigher concentration of one or more non-Nb elements therewithin mayincrease the mechanical strength and/or diffusion resistance of theremaining, thinner diffusion barrier notwithstanding its reducedthickness.

In various embodiments, the diffusion barrier may be a multi-layerannular structure in which one or more of the layers include, consistessentially of, or consist essentially of an Nb alloy as detailedherein, and one or more other layers include, consist essentially of, orconsist of Nb (or an Nb alloy containing a lesser concentration of oneor more of the non-Nb alloying elements; references herein to “Nblayers” or “layers of Nb” include such layers). For example, thediffusion barrier may include, consist essentially of, or consist of aninner layer of Nb surrounded by an outer layer of the Nb alloy, or viceversa. In another embodiment, the diffusion barrier may include, consistessentially of, or consist of a layer of the Nb alloy sandwiched betweenan inner layer of Nb and an outer layer of Nb. As detailed herein,during heat treatment, all or a portion of the Nb layer(s) of thediffusion barrier may be converted into the superconducting phase, whilethe Nb-alloy layer(s) remain unconverted.

Embodiments of the present invention may also incorporate stabilizingelements within the wires themselves and/or within the compositefilaments utilized to form the wire. For example, embodiments of theinvention may incorporate stabilizing elements that include, consistessentially of, or consist of Ta, a Ta alloy (e.g., an alloy of Ta and Wsuch as Ta-3W), or an alloy of Nb with one or more of Hf, Ti, Zr, Ta, V,Y, Mo, or W, as described in U.S. patent application Ser. No.15/205,804, filed on Jul. 8, 2016 (“the '804 application”), the entiredisclosure of which is incorporated by reference herein. Insuperconducting wires in accordance with the present invention,stabilizing elements are typically separated from monofilaments and/orcomposite filaments via one or more diffusion barriers therebetween.

In an aspect, embodiments of the invention feature a superconductingwire that includes, consists essentially of, or consists of an outerwire matrix, a diffusion barrier disposed within the wire matrix, and aplurality of composite filaments surrounded by the diffusion barrier andseparated from the outer wire matrix by the diffusion barrier. The outerwire matrix includes, consists essentially of, or consists of Cu. Thediffusion barrier includes, consists essentially of, or consists of aNb—W alloy (e.g., a Nb alloy containing 0.1%-20% W or 0.2%-12% W or0.2%-10% W) or a Nb—Ta—W alloy. One or more, or even each, of thecomposite filaments includes, consists essentially of, or consists of(i) a plurality of monofilaments and (ii) a cladding surrounding theplurality of monofilaments. The composite-filament cladding may include,consist essentially of, or consist of Cu. One or more, or even each, ofthe monofilaments includes, consists essentially of, or consists of acore and a cladding surrounding the core. The monofilament core mayinclude, consist essentially of, or consist of Nb. The monofilamentcladding may include, consist essentially of, or consist of Cu. Thediffusion barrier extends through an axial dimension of thesuperconducting wire.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The diffusion barrier may occupy lessthan approximately 20% of a cross-section of the wire, less thanapproximately 15% of a cross-section of the wire, less thanapproximately 10% of a cross-section of the wire, or less thanapproximately 5% of a cross-section of the wire. The diffusion barriermay occupy more than approximately 1% of a cross-section of the wire,more than approximately 2% of a cross-section of the wire, more thanapproximately 5% of a cross-section of the wire, more than approximately8% of a cross-section of the wire, or more than approximately 10% of across-section of the wire. The wire may include an annular region orlayer disposed proximate the diffusion barrier (e.g., on either or bothsides thereof, for example, disposed between the composite filaments andthe diffusion barrier), and at least a portion of the annular region mayinclude, consist essentially of, or consist of a Nb-basedsuperconducting phase (e.g., Nb—Ti and/or Nb₃Sn). A portion of theannular region may include, consist essentially of, or consist of a Nballoy or Nb—Ta alloy having a composition different from that of thediffusion barrier. The annular region may conform to and/or be in directmechanical contact with the diffusion barrier.

The core of one or more, or even each, of the monofilaments may include,consist essentially of, or consist of an alloy, pseudo-alloy, or mixturecontaining Nb and one or more of Ti, Zr, Hf, Ta, Y, or La (e.g., Nb—Ti).The core of one or more, or even each, of the monofilaments may include,consist essentially of, or consist of Nb₃Sn. The diffusion barrier mayinclude, consist essentially of, or consist of Nb-3W or Nb-6W or Nb-12W.The diffusion barrier may additionally contain one or more alloyingelements selected from the group consisting of Ru, Pt, Pd, Rh, Os, Ir,Mo, Re, or Si. The cross-sectional thickness and/or cross-sectional areaof the diffusion barrier may be substantially constant along thethickness of the wire. One or more, or even each, of the compositefilaments may have a hexagonal cross-sectional shape (i.e., incross-section perpendicular to the axial dimension of the wire). One ormore, or even each, of the monofilaments may have a hexagonalcross-sectional shape (i.e., in cross-section perpendicular to the axialdimension of the wire).

The wire may include a stabilizing element disposed within the pluralityof composite filaments and surrounded by the diffusion barrier. Thestabilizing element may include, consist essentially of, or consist ofCu and/or a Ta alloy containing 0.1%-20% W or 0.2%-12% W or 0.2%-10% W.At least a portion of the stabilizing element may be locatedsubstantially at the central core of the superconducting wire. Thestabilizing element may occupy less than approximately 20% of across-section of the wire, less than approximately 15% of across-section of the wire, less than approximately 10% of across-section of the wire, or less than approximately 5% of across-section of the wire. The stabilizing element may occupy more thanapproximately 1% of a cross-section of the wire, more than approximately2% of a cross-section of the wire, more than approximately 5% of across-section of the wire, more than approximately 8% of a cross-sectionof the wire, or more than approximately 10% of a cross-section of thewire.

In another aspect, embodiments of the invention feature asuperconducting wire that includes, consists essentially of, or consistsof a wire matrix and a plurality of composite filaments embedded withinthe wire matrix. The wire matrix includes, consists essentially of, orconsists of Cu. One or more, or even each, of the composite filamentsincludes, consists essentially of, or consists of (i) a plurality ofmonofilaments, (ii) a diffusion barrier extending through an axialdimension of the composite filament and surrounding the plurality ofmonofilaments, and (iii) a cladding surrounding the diffusion barrier,the diffusion barrier separating the cladding from the plurality ofmonofilaments. The composite-filament diffusion barrier includes,consists essentially of, or consists of a Nb—W alloy or a Nb—Ta—W (e.g.,a Nb alloy or a Nb—Ta alloy containing 0.1%-20% W or 0.2%-12% W or0.2%-10% W). The composite-filament cladding includes, consistsessentially of, or consists of Cu. One or more, or even each, of themonofilaments includes, consists essentially of, or consists of a coreand a cladding surrounding the core. The monofilament core may include,consist essentially of, or consist of Nb. The monofilament cladding mayinclude, consist essentially of, or consist of Cu.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The diffusion barriers maycollectively occupy less than approximately 20% of a cross-section ofthe wire, less than approximately 15% of a cross-section of the wire,less than approximately 10% of a cross-section of the wire, or less thanapproximately 5% of a cross-section of the wire. The diffusion barriersmay collectively occupy more than approximately 1% of a cross-section ofthe wire, more than approximately 2% of a cross-section of the wire,more than approximately 5% of a cross-section of the wire, more thanapproximately 8% of a cross-section of the wire, or more thanapproximately 10% of a cross-section of the wire. The wire may includean annular region or layer disposed proximate at least one diffusionbarrier (e.g., on either or both sides thereof, for example, disposedbetween the monofilaments and the diffusion barrier of at least one ofthe composite filaments), and at least a portion of the annular regionmay include, consist essentially of, or consist of a Nb-basedsuperconducting phase (e.g., Nb—Ti and/or Nb₃Sn). A portion of theannular region may include, consist essentially of, or consist of a Nballoy or Nb—Ta alloy having a composition different from that of thediffusion barrier. The annular region may conform to and/or be in directmechanical contact with the diffusion barrier.

The core of one or more, or even each, of the monofilaments may include,consist essentially of, or consist of an alloy, pseudo-alloy, or mixturecontaining Nb and one or more of Ti, Zr, Hf, Ta, Y, or La (e.g., Nb—Ti).The core of one or more, or even each, of the monofilaments may include,consist essentially of, or consist of Nb₃Sn. The diffusion barrier mayinclude, consist essentially of, or consist of Nb-3W, Nb-6W, or Nb-12W.The diffusion barrier may additionally contain one or more alloyingelements selected from the group consisting of Ru, Pt, Pd, Rh, Os, Ir,Mo, Re, or Si. The cross-sectional thickness and/or cross-sectional areaof the diffusion barrier may be substantially constant along thethickness of the wire. One or more, or even each, of the compositefilaments may have a hexagonal cross-sectional shape (i.e., incross-section perpendicular to the axial dimension of the wire). One ormore, or even each, of the monofilaments may have a hexagonalcross-sectional shape (i.e., in cross-section perpendicular to the axialdimension of the wire).

The wire may include a stabilizing element disposed within the pluralityof composite filaments. The stabilizing element may include, consistessentially of, or consist of Cu and/or a Ta alloy containing 0.1%-20% Wor 0.2%-12% W or 0.2%-10% W. At least a portion of the stabilizingelement may be located substantially at the central core of thesuperconducting wire. The stabilizing element may occupy less thanapproximately 20% of a cross-section of the wire, less thanapproximately 15% of a cross-section of the wire, less thanapproximately 10% of a cross-section of the wire, or less thanapproximately 5% of a cross-section of the wire. The stabilizing elementmay occupy more than approximately 1% of a cross-section of the wire,more than approximately 2% of a cross-section of the wire, more thanapproximately 5% of a cross-section of the wire, more than approximately8% of a cross-section of the wire, or more than approximately 10% of across-section of the wire.

In yet another aspect, embodiments of the invention feature asuperconducting wire that includes, consists essentially of, or consistsof an inner wire stabilizing matrix, a diffusion barrier disposed aroundthe wire stabilizing matrix, and a plurality of composite filamentsdisposed around the diffusion barrier and separated from the wirestabilizing matrix by the diffusion barrier. The wire stabilizing matrixincludes, consists essentially of, or consists of Cu. The diffusionbarrier includes, consists essentially of, or consists of a Nb—W alloyor a Nb—Ta—W alloy (e.g., a Nb alloy or a Nb—Ta alloy containing0.1%-20% W or 0.2%-12% W or 0.2%-10% W). One or more, or even each, ofthe composite filaments includes, consists essentially of, or consistsof (i) a plurality of monofilaments, and (ii) (iii) a claddingsurrounding the plurality of monofilaments. The composite-filamentcladding includes, consists essentially of, or consists of Cu. Thediffusion barrier extends through an axial dimension of the wire.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The diffusion barrier may occupy lessthan approximately 20% of a cross-section of the wire, less thanapproximately 15% of a cross-section of the wire, less thanapproximately 10% of a cross-section of the wire, or less thanapproximately 5% of a cross-section of the wire. The diffusion barriermay occupy more than approximately 1% of a cross-section of the wire,more than approximately 2% of a cross-section of the wire, more thanapproximately 5% of a cross-section of the wire, more than approximately8% of a cross-section of the wire, or more than approximately 10% of across-section of the wire. The wire may include an annular region orlayer disposed proximate the diffusion barrier (e.g., on either or bothsides thereof, for example, disposed between the composite filaments andthe diffusion barrier), and at least a portion of the annular region mayinclude, consist essentially of, or consist of a Nb-basedsuperconducting phase (e.g., Nb—Ti and/or Nb₃Sn). A portion of theannular region may include, consist essentially of, or consist of a Nballoy or Nb—Ta alloy having a composition different from that of thediffusion barrier. The annular region may conform to and/or be in directmechanical contact with the diffusion barrier.

The core of one or more, or even each, of the monofilaments may include,consist essentially of, or consist of an alloy, pseudo-alloy, or mixturecontaining Nb and one or more of Ti, Zr, Hf, Ta, Y, or La (e.g., Nb—Ti).The core of one or more, or even each, of the monofilaments may include,consist essentially of, or consist of Nb₃Sn. The diffusion barrier mayinclude, consist essentially of, or consist of Nb-3W or Nb-6W or Nb-12W.The diffusion barrier may additionally contain one or more alloyingelements selected from the group consisting of Ru, Pt, Pd, Rh, Os, Ir,Mo, Re, or Si. The cross-sectional thickness and/or cross-sectional areaof the diffusion barrier may be substantially constant along thethickness of the wire. One or more, or even each, of the compositefilaments may have a hexagonal cross-sectional shape (i.e., incross-section perpendicular to the axial dimension of the wire). One ormore, or even each, of the monofilaments may have a hexagonalcross-sectional shape (i.e., in cross-section perpendicular to the axialdimension of the wire).

The wire may include a stabilizing element disposed within the pluralityof composite filaments or within or proximate the inner wire stabilizingmatrix. The stabilizing element may include, consist essentially of, orconsist of Cu and/or a Ta alloy containing 0.1%-20% W or 0.2%-12% W or0.2%-10% W. At least a portion of the stabilizing element may be locatedsubstantially at the central core of the superconducting wire. Thestabilizing element may occupy less than approximately 20% of across-section of the wire, less than approximately 15% of across-section of the wire, less than approximately 10% of across-section of the wire, or less than approximately 5% of across-section of the wire. The stabilizing element may occupy more thanapproximately 1% of a cross-section of the wire, more than approximately2% of a cross-section of the wire, more than approximately 5% of across-section of the wire, more than approximately 8% of a cross-sectionof the wire, or more than approximately 10% of a cross-section of thewire.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “approximately” and “substantially” mean±10%, and in someembodiments, ±5%. The term “consists essentially of” means excludingother materials that contribute to function, unless otherwise definedherein. Nonetheless, such other materials may be present, collectivelyor individually, in trace amounts. For example, a structure consistingessentially of multiple metals will generally include only those metalsand only unintentional impurities (which may be metallic ornon-metallic) that may be detectable via chemical analysis but do notcontribute to function (and may be present at concentrations of, e.g.,less than 5 ppm, 2 ppm, 1 ppm, 0.5 ppm, or 0.1 ppm). As used herein,“consisting essentially of at least one metal” refers to a metal or amixture of two or more metals but not compounds between a metal and anon-metallic element or chemical species such as oxygen, silicon, ornitrogen (e.g., metal nitrides, metal silicides, or metal oxides); suchnon-metallic elements or chemical species may be present, collectivelyor individually, in trace amounts, e.g., as impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A is a schematic cross-sectional view of a tube utilized to form amonofilament in accordance with various embodiments of the invention;

FIG. 1B is a schematic cross-sectional view of a rod utilized to form amonofilament in accordance with various embodiments of the invention;

FIG. 1C is a schematic cross-sectional view of a monofilament utilizedto form a composite filament in accordance with various embodiments ofthe invention;

FIG. 2A is a schematic cross-sectional view of a tube utilized to form acomposite filament in accordance with various embodiments of theinvention;

FIG. 2B is a schematic cross-sectional view of a tube utilized to form adiffusion barrier within a composite filament in accordance with variousembodiments of the invention;

FIG. 2C is a schematic cross-sectional view of a stack of monofilamentsutilized to form a composite filament in accordance with variousembodiments of the invention;

FIG. 2D is a schematic cross-sectional view of a composite filament atan initial stage of fabrication in accordance with various embodimentsof the invention;

FIG. 2E is a schematic cross-sectional view of a composite filamentutilized to form superconducting wires in accordance with variousembodiments of the invention;

FIG. 3A is a schematic cross-sectional view of a tube utilized to form astabilizing element in accordance with various embodiments of theinvention;

FIG. 3B is a schematic cross-sectional view of a rod utilized to form astabilizing element in accordance with various embodiments of theinvention;

FIG. 3C is a schematic cross-sectional view of a stabilizing elementutilized to form stabilized composite filaments and/or superconductingwires in accordance with various embodiments of the invention;

FIG. 3D is a schematic cross-sectional view of a composite filamentincorporating a stabilizing element in accordance with variousembodiments of the invention;

FIG. 4A is a schematic cross-sectional view of a tube utilized to form asuperconducting wire in accordance with various embodiments of theinvention;

FIG. 4B is a schematic cross-sectional view of a stack of compositefilaments utilized to form a superconducting wire in accordance withvarious embodiments of the invention;

FIG. 4C is a schematic cross-sectional view of a tube utilized to form adiffusion barrier within a superconducting wire in accordance withvarious embodiments of the invention;

FIG. 4D is a schematic cross-sectional view of a superconducting wire atan initial stage of fabrication in accordance with various embodimentsof the invention;

FIG. 4E is a schematic cross-sectional view of a superconducting wire inaccordance with various embodiments of the invention;

FIG. 4F is a schematic cross-sectional view of a stabilizedsuperconducting wire at an initial stage of fabrication in accordancewith various embodiments of the invention;

FIG. 4G is a schematic cross-sectional view of a stabilizedsuperconducting wire in accordance with various embodiments of theinvention;

FIG. 5 is a cross-sectional micrograph of a superconducting wirefeaturing a Cu inner stabilizer and a diffusion barrier disposed aroundthe stabilizer in accordance with various embodiments of the invention;and

FIG. 6 is a cross-sectional micrograph of a superconducting wirefeaturing a Cu outer matrix and a diffusion barrier disposed between theouter matrix and the wire filaments in accordance with variousembodiments of the invention.

DETAILED DESCRIPTION

FIGS. 1A-1C depict components of an exemplary monofilament 100 andconstituent components thereof. In accordance with embodiments of theinvention, a rod 105 is disposed within a tube 110 that includes,consists essentially of, or consists of Cu or a Cu alloy (e.g., bronze).The composition of the rod 105 may be selected based on the particularmetallic superconductor desired in the final wire. For example, the rod105 may include, consist essentially of, or consist of Nb, Ti, Nb—Ti, oran alloy thereof. In other examples, the rod 105 may include, consistessentially of, or consist of Nb alloyed with one or more of Ti, Zr, Hf,Ta, Y, or La. Such alloying elements may be individually or collectivelypresent within the rod 105 (and thus within the core of a monofilament100) is concentrations of, for example, 0.2%-10% (e.g., 0.2%-5%, or0.5%-1%). In various embodiments, the tube 110 (and/or any other tubedescribed herein) may be formed by wrapping a metallic sheet around therod 105; in such embodiments, the ends of the sheet may overlap. The rod105 clad with the tube 110 may subsequently be drawn down to reduce itsdiameter to, for example, between 0.5 inch and 1.5 inches. The clad rodmay be drawn down in multiple stages and may be heat treated duringand/or after any or each of the drawing steps for, e.g., strain relief.Once drawn down, the clad rod may be drawn through a shaped die in orderto fabricate the monofilament 100 shaped for efficient stacking withother monofilaments. For example, as shown in FIG. 1C, a hexagonal diemay be utilized to form a monofilament 100 having a hexagonalcross-section. In other embodiments, monofilaments may have othercross-sections, e.g., square, rectangular, triangular, etc. As shown inFIG. 1C, a monofilament 100 typically includes, consists essentially of,or consists of a single annular cladding disposed around and surroundinga single cylindrical core having a substantially uniform composition;thus, regions of a superconducting wire in accordance with embodimentsof the invention containing multiple clad and separated cylindricalcores correspond to multiple “monofilaments” or a single “compositefilament.”

Once a monofilament 100 is fabricated, other monofilaments 100 may alsobe fabricated in the same manner, or one or more monofilaments 100 maybe divided into multiple pieces. Multiple monofilaments may be stackedtogether to form at least a portion of a composite filament. FIGS. 2A-2Edepict various components and assembly of a composite filament 200. Asshown in FIG. 2C, multiple monofilaments 100 may be stacked together inan arrangement that will subsequently become at least a portion of thecore of composite filament 200. While FIG. 2C depicts the stacking of 19different monofilaments 100, embodiments of the invention may includemore or fewer monofilaments 100. The stacked assembly of monofilaments100 may be disposed within a tube 205 that includes, consistsessentially of, or consists of Cu or a Cu alloy (e.g., bronze). As shownin FIG. 2B, a tube 210 may be disposed within the tube 205 and aroundthe stack of monofilaments 100; this tube 210 will become the diffusionbarrier 215 in the final composite filament and retard or substantiallyprevent interdiffusion between the monofilaments 100 and the material oftube 205, which becomes the outer matrix 220 of the resulting compositefilament. Thus, the tube 210 may include, consist essentially of, orconsist of a Nb alloy or a Nb—Ta alloy such as Nb—W (e.g., Nb-12W orNb-6W or Nb-3W) or Nb—Ta—W (e.g., Nb—Ta-12W or Nb—Ta-6W or Nb—Ta-3W).Before and/or after the monofilaments 100 are disposed within the tube205 and the tube 210, the monofilaments 100, the tube 205, and/or thetube 210 may be cleaned and/or etched (e.g., via a cleaning agentincluding, consisting essentially of, or consisting of one or moreacids) to, for example, remove surface oxides and/or other contaminants.

The tube 210 may be fabricated via alloying of pure Nb or a Nb—Ta alloywith the one or more other alloying elements disposed within thediffusion barrier. For example, for diffusion barriers (and thus tubes210) including, consisting essentially of, or consisting of an alloy ofNb and W, Nb and W may be alloyed together in the desired amounts via aprocess such as electron-beam melting and/or arc melting. Similarly, fordiffusion barriers (and thus tubes 210) including, consistingessentially of, or consisting of an alloy of Nb, Ta, and W, Nb, Ta, andW may be alloyed together in the desired amounts via a process such aselectron-beam melting and/or arc melting. The resulting material may befabricated into a sheet, and the sheet may be formed into a tube by,e.g., rolling, deep drawing, extrusion, pilgering, etc.

As shown in FIG. 2D, the tube 205 and tube 210 may be compacted onto themonofilaments 100 by, e.g., swaging, extruding, and/or rolling. The cladstacked monofilaments 100 may be annealed to promote bonding between thevarious monofilaments 100 in the stacked assembly. For example, the cladstacked monofilaments may be annealed at a temperature betweenapproximately 300° C. and approximately 500° C. (e.g., approximately400° C.) for a time of approximately 0.5 hour and approximately 3 hours(e.g., approximately 1 hour). Advantageously, the presence of thediffusion barrier 215 between the monofilaments 100 and the outer matrix220 substantially prevents diffusion between the Cu of the matrix 220and the monofilaments 100, thereby preventing the formation of metallicphases having low electrical conductivity (e.g., electrical conductivitylower than Cu and/or than the material of matrix 220). The diffusionbarrier 215 also provides additional mechanical strength to the finalwire, given its superior mechanical properties (e.g., strength, yieldstrength, tensile strength, stiffness, Young's modulus, etc.) comparedwith those of the outer matrix 220 and/or the monofilaments 100,particularly after the extended high-temperature heat treatmentsutilized for reactive formation of the superconducting phase in thewire.

The resulting assembly may be drawn down one or more times to reduce itsdiameter, and may subsequently be drawn through a shaped die in order toprovide composite filament 200 with a cross-sectional shape configuredfor efficient stacking. For example, as shown in FIG. 2E, a hexagonaldie may be utilized to form a composite filament 200 having a hexagonalcross-section. In other embodiments, composite filaments 200 may haveother cross-sections, e.g., square, rectangular, triangular, round,off-round, elliptical, etc. In various embodiments, the cross-sectionalsize and/or shape of the composite filament 200 after processing andshaping is equal to the cross-sectional size and/or shape of themonofilament 100 utilized in the initial stacked assembly before beingreduced in size (i.e., shown in FIG. 2C). (Although the diffusionbarrier 215 resulting from the incorporation of tube 210 is depicted inFIGS. 2D and 2E as having a variable cross-sectional thickness, invarious embodiments of the invention the diffusion barrier 215 has asubstantially uniform cross-sectional thickness around itscircumference, and diffusion barrier 215 may have the form, incross-section, of an annular ring (for example, a ring disposed tightlyaround the filaments (or other structures) therewithin), as shown inFIGS. 5 and 6; diffusion barriers having an annular cross-section inaccordance with embodiments of the invention generally have the form ofcylinders that extend along the axial dimension of the wire.)

Superconducting wires in accordance with embodiments of the inventionmay also incorporate stabilizing elements that provide even moremechanical strength while not compromising the drawability and/orelectrical performance of the wires. FIGS. 3A-3C depict the fabricationof a stabilizing element 300 via a method similar to that detailed abovefor monofilaments 100. In accordance with embodiments of the invention,a rod 305 is disposed within a tube 310 that includes, consistsessentially of, or consists of Cu or a Cu alloy. The rod 305 mayinclude, consist essentially of, or consist of one or more metals havingmechanical strength (e.g., tensile strength, yield strength, etc.)greater than that of rods 105 utilized to fabricate monofilaments 100.For example, the rod 305 may include, consist essentially of, or consistof Ta or a Ta alloy (e.g., a Ta—W alloy such as Ta-3W), Nb or a Nb alloy(e.g., a Nb—W alloy such as Nb-12W, Nb-6W, or Nb-3W, a Nb—Ta alloy, aNb—Ta alloy containing one or more additional alloying elements such asHf, Ti, Zr, Ta, V, Y, Mo, or W, or any other material disclosed hereinas suitable for diffusion barriers. In other embodiments, the rod 305may include, consist essentially of, or consist of a Nb alloy havinggreater mechanical strength than substantially pure Nb. For example,rods 305 (and therefore stabilizing elements) in accordance withembodiments of the invention may include, consist essentially of, orconsist of an alloy of Nb with one or more of Hf, Ti, Zr, Ta, V, Y, Mo,or W. For example, stabilizing elements in accordance with embodimentsof the invention may include, consist essentially of, or consist of NbC103 alloy, which includes approximately 10% Hf, approximately 0.7%-1.3%Ti, approximately 0.7% Zr, approximately 0.5% Ta, approximately 0.5% W,and the balance Nb. In other embodiments, stabilizing elements mayinclude, consist essentially of, or consist of a Nb B66 alloy and/or aNb B77 alloy.

The rod 305 clad with the tube 310 may subsequently be drawn down toreduce its diameter to, for example, between 0.5 inch and 1.5 inches.The clad rod may be drawn down in multiple stages and may be heattreated during and/or after any or each of the drawing steps for, e.g.,strain relief. Once drawn down, the clad rod may be drawn through ashaped die in order to fabricate the stabilizing element 300 shaped forefficient stacking with monofilaments 100 and/or composite filaments200. For example, as shown in FIG. 3C, a hexagonal die may be utilizedto form a stabilizing element 300 having a hexagonal cross-section. Inother embodiments, stabilizing elements 300 may have othercross-sections, e.g., square, rectangular, triangular, etc. In variousembodiments, stabilizing elements 300 may have cross-sectional sizesand/or shapes substantially the same as cross-sectional sizes and/orshapes of monofilaments 100 and/or composite filaments 200.

Once fabricated, one or more stabilizing elements 300 may be insertedinto a stack of monofilaments 100, and the resulting assembly may besurrounded with a diffusion-barrier material and a matrix material,drawn down, and optionally shaped to form a stabilized compositefilament 315 (e.g., as described above with reference to FIGS. 2A-2E)that incorporates a diffusion barrier 215 between the monofilaments 100and stabilizing element(s) 300 and the outer matrix 220, as shown inFIG. 3D. In various embodiments of the invention, composite filament mayinclude a diffusion barrier between the stabilizing element 300 and theremaining monofilaments 100 in order to retard or substantially preventinterdiffusion therebetween. In various embodiments, the stabilizingelement 300 may be replaced or supplemented with an internal stabilizingmatrix that includes, consists essentially of, or consists of, e.g., Cuor a Cu alloy, and such regions may be separated from monofilaments 100via one or more diffusion barriers. Although FIG. 3D depicts thestabilizing element 300 as having a cross-sectional area substantiallythe same as that of one of the monofilaments 100, in various embodimentsof the invention, the stabilizing element 300 has a cross-sectional arealarger than that of a single monofilament 100. For example, thecross-sectional area of the stabilizing element 300 may be at least 1.5times, at least 2 times, at least 3 times, at least 4 times, at least 5times, or at least 6 times the cross-sectional area of a monofilament100.

In embodiments of the invention incorporating stabilizing elements aswell as diffusion barriers, the amount of cross-sectional area of thewire imparting additional mechanical strength may be beneficiallydivided between the diffusion barrier(s) and the stabilizing element(s).That is, the more cross-sectional area of the wire occupied by one ormore stabilizing elements, the less cross-sectional area of the wireneed be occupied by the diffusion barrier(s), as long as each diffusionbarrier has sufficient thickness to retard or substantially eliminatediffusion between the various portions of the wire. Conversely, the useof diffusion barriers in accordance with embodiments of the inventionenables the use of one or more stabilizing elements that themselvescollectively occupy less of the cross-sectional area of the wire whilestill imparting the desired mechanical strength (and/or other mechanicalproperties) to the wire. In various embodiments, the diffusionbarrier(s) may collectively occupy at least 1%, at least 2%, at least3%, at least 4%, or at least 5% of the cross-sectional area of the wire.In various embodiments, the diffusion barrier(s) may collectively occupyless than 15%, less than 12%, less than 10%, less than 9%, less than 8%,less than 7%, less than 6%, or less than 5% of the cross-sectional areaof the wire. In embodiments of the invention featuring stabilizingelements, the stabilizing elements and diffusion barriers maycollectively occupy less than 25%, less than 20%, less than 15%, or lessthan 10% of the cross-sectional area of the wire. Stabilizing elementsthemselves may occupy less than 15% or less than 10% (e.g.,approximately 2% to approximately 8%, or approximately 5% toapproximately 15%) of the cross-sectional area of the wire. Stabilizingelements may occupy at least 2%, at least 3%, at least 5%, or at least8% of the cross-sectional area of the wire.

In addition to or instead of being incorporated within one or morecomposite filaments 200, 315, diffusion barriers in accordance withembodiments of the present invention may be disposed between an outerstabilizing matrix (and/or an inner stabilizing matrix and/or stabilizerproximate the center of the wire) and the composite filaments toadvantageously retard or substantially prevent interdiffusion within thesuperconducting wire. That is, superconducting wires and/or wirepreforms may be fabricated utilizing diffusion barriers disposed aroundassemblies of composite filaments 200, stabilized composite filaments315, and/or composite filaments lacking their own diffusion barriers.FIGS. 4A-4E depict various stages of the fabrication of an exemplarysuperconducting wire 400. As shown in FIG. 4B, multiple compositefilaments 405 each lacking their own internal diffusion barriers may bestacked together in an arrangement that will subsequently become atleast a portion of the core of superconducting wire 400. Each compositefilament 405 may be fabricated, for example, similarly to compositefilament 200 detailed above but without incorporation of the diffusionbarrier 215 arising from the use of tube 210 during fabrication. Inother embodiments, the stack of composite filaments may include or becomposed of composite filaments 200, composite filaments 315, and/ormixtures thereof with or without composite filaments 405. While FIG. 4Bdepicts the stacking of 18 different composite filaments 405,embodiments of the invention may include more or fewer compositefilaments.

The stacked assembly of composite filaments may be disposed within atube 410 that includes, consists essentially of, or consists of Cu or aCu alloy. In addition, as shown in FIG. 4C, a tube 210 may be disposedaround the stacked assembly of composite filaments and within the tube410 and may therefore form a diffusion barrier in the final wire. Beforeand/or after the composite filaments are disposed within the tube 510and the tube 210, the composite filaments, the tube 210, and/or the tube410 may be cleaned and/or etched (e.g., via a cleaning agent including,consisting essentially of, or consisting of one or more acids) to, forexample, remove surface oxides and/or other contaminants. As shown inFIG. 4D, the tube 410 and the tube 210 may be compacted onto thecomposite filaments by, e.g., swaging, extruding, and/or rolling, andtube 210 may become diffusion barrier 415, and tube 410 may become outermatrix 420. The clad stacked composite filaments may be annealed topromote bonding between the various composite filaments in the stackedassembly. For example, the clad stack may be annealed at a temperaturebetween approximately 300° C. and approximately 500° C. (e.g.,approximately 400° C.) for a time of approximately 0.5 hour andapproximately 3 hours (e.g., approximately 1 hour). Advantageously, thepresence of the diffusion barrier 415 between the composite filaments405 and the outer matrix 420 substantially prevents diffusion betweenthe Cu of the matrix 420 and the composite filaments 405, therebypreventing the formation of metallic phases having low electricalconductivity (e.g., electrical conductivity lower than Cu and/or thanthe material of matrix 420). The resulting assembly may be drawn downone or more times to reduce its diameter, as shown in FIG. 4E. Before orafter drawing, the superconducting wire 400 may be annealed to, e.g.,relax residual stresses and/or promote recrystallization therein.

As shown in FIGS. 4F and 4G, a similar methodology may be utilized tofabricate stabilized superconducting wires 425 that incorporate one ormore diffusion barriers 415 as well as one or more stabilizing elements300. For example, the assembly of stacked composite filaments may definetherewithin one or more voids each sized and shaped to accommodate oneor more stabilizing elements 300. Before or after the compositefilaments are disposed within the tube 410 and the tube 210, one or morestabilizing elements 300 may be disposed within each of the voids, asshown in FIG. 4F. The resulting assembly may have its diameter reducedby, e.g., drawing and/or extrusion, as shown in FIG. 4G. In variousembodiments, a diffusion barrier may be disposed between stabilizingelement(s) 300 and the filaments within the wire or wire preform,particularly in embodiments in which the stabilizing element includes,consists essentially of, or consists of Cu. For example, a tube of thedesired diffusion barrier material may be disposed around thestabilizing element when the wire preform assembly is assembled, and theentire assembly may be drawn down to the desired wire dimensions. WhileFIGS. 4F and 4G depicts the superconducting wire 425 having a singlestabilizing element 300 disposed substantially at the center of thestacked assembly of composite filaments, in accordance with embodimentsof the invention, one or more stabilizing elements 300 may be disposedat other locations within the stacked assembly in addition to or insteadof the stabilizing element 300 disposed at the center. Although FIGS. 4Fand 4G depict the stabilizing element 300 as having a cross-sectionalarea substantially the same as that of one of the composite filaments405, in various embodiments of the invention, the stabilizing element300 has a cross-sectional area larger than that of a single compositefilament 405. For example, the cross-sectional area of the stabilizingelement 300 may be at least 1.5 times, at least 2 times, at least 3times, at least 4 times, at least 5 times, or at least 6 times thecross-sectional area of a composite filament 405.

In various embodiments, the superconducting wire 400, 425 lacks adiffusion barrier 415 therewithin, and thus, tube 210 is not utilized information thereof, and diffusion barriers 215 in one or more of theindividual composite filaments are utilized to retard or substantiallyprevent interdiffusion. In other embodiments, as shown in FIG. 4D-4G,the individual composite filaments 405 may lack diffusion barrierstherewithin, and diffusion barrier 415 is present within thesuperconducting wire 400, 425. In such embodiments, the tubes 110 and/or205 may incorporate therewithin Sn which advantageously reacts with theNb of the filaments during subsequent thermal processing to form asuperconducting phase (e.g., Nb₃Sn). In other embodiments, diffusionbarrier 415 is present in addition to diffusion barriers 215 within theindividual composite filaments.

In various embodiments, the superconducting wire 400, superconductingwire 425, composite filament 4015, composite filament 200, and/orstabilized composite filament 315 may be mechanically processed fordiameter reduction and/or to promote bonding between their constituentelements prior to wire drawing steps. For example, the superconductingwire 400, superconducting wire 425, composite filament 4015, compositefilament 200, and/or stabilized composite filament 315 may be extruded,swaged, and/or rolled prior to the final drawing step(s). In variousembodiments, the superconducting wire 400, superconducting wire 425,composite filament 4015, composite filament 200, and/or stabilizedcomposite filament 315 may be heat treated during and/or after each ofmultiple different drawing steps for strain relief. For example, duringand/or after one or more of the drawing steps, the superconducting wire400, superconducting wire 425, composite filament 4015, compositefilament 200, and/or stabilized composite filament 315 may be annealedat temperatures from approximately 360° C. to approximately 420° C. fora time period of, e.g., approximately 20 hours to approximately 40hours.

In various embodiments of the present invention, the superconductingwire 400 or superconducting wire 425 may be cooled below the criticaltemperature of the filaments therewithin and utilized to conductelectrical current. In some embodiments, multiple superconducting wires400 and/or superconducting wires 425 are coiled together to form asingle superconducting cable.

While some superconducting wires 400, 425 (e.g., those incorporatingNb—Ti-containing filaments) may be utilized directly in superconductingapplications, the fabrication processes for various othersuperconducting wires 400, 425 may incorporate one or more steps toincorporate a portion of the superconducting phase. For example, Nb₃Snsuperconducting phases, once formed, are typically brittle and may notbe further drawn or otherwise mechanically deformed without damage.Thus, embodiments of the present invention may be utilized to fabricatesuperconducting wires 400, 425 that incorporate Nb and Sn separate fromeach other; once the wires 400, 425 are mostly or fully fabricated, thewires 400, 425 may be annealed to interdiffuse the Nb and Sn and formthe superconducting Nb₃Sn phase therewithin. For example, the drawn wiremay be annealed at temperatures from approximately 600° C. toapproximately 700° C. for a time period of, e.g., approximately 30 hoursto approximately 200 hours. In various embodiments, one or more of theCu-based tubes 110, 205, or 310 may incorporate Sn therewithin; e.g.,one or more of the tubes may include, consist essentially of, or consistof a Cu—Sn alloy (that includes, e.g., 13-15% Sn). Such materials areductile, enabling the fabrication of the various filaments and wires asdetailed herein. Thereafter, the wire 400, 425 may be annealed,resulting in interdiffusion and the formation of the superconductingNb₃Sn phase at least at the interface between the Nb and the Cu—Sn.

In other embodiments, pure Sn or an Sn alloy (e.g., an Sn alloy with Cuor magnesium (Mg)) may be incorporated (e.g., in the form of a rod ortube) within one or more of the stacks utilized to form compositefilaments 200, stabilized composite filaments 315, and/or wires 400,425; after formation of the composite filaments 200, stabilizedcomposite filaments 315, and/or wires 400, 425 as detailed herein, anannealing step may be performed to form the superconducting Nb₃Sn phase.

In various embodiments, at least the Nb within a portion of one or morediffusion barriers within a wire reacts as described above to form asuperconducting phase, and this reacted portion of the diffusion barriermay therefore contribute to the superconductive conductivity of the wireduring operation. For example, an inner or outer portion of a diffusionbarrier may react with, e.g., Sn or an Sn alloy, to form asuperconducting phase substantially identical or similar to that formedfrom the filaments of the wire. In such embodiments, the thickness ofthe diffusion barrier is typically sufficiently large such that theentirety of the diffusion barrier does not react to form asuperconducting phase. Thus, at least a portion of the diffusion barrierremains unreacted and contributes resistance to interdiffusion as wellas mechanical strength to the wire, as described herein. In variousembodiments, the diffusion barrier may be a multi-layer structurecontaining one or more annular layers including, consisting essentiallyof, or consisting of Nb as well as one or more annular layers including,consisting essentially of, or consisting of a Nb alloy or Nb—Ta alloy asdetailed herein. The alloy layer(s) may provide most of the diffusionresistance, while at least a portion of the Nb layer(s) may react (withsurrounding Sn in the Cu matrix, for example) during heat treatment tobecome part of the superconducting phase. For example, the diffusionbarrier may include, consist essentially of, or consist of an alloylayer sandwiched between two different Nb layers. In another example, aninner Nb layer may be surrounded by an outer alloy layer or vice versa.

FIG. 5 is a cross-sectional view of a superconducting wire 500incorporating a diffusion barrier in accordance with embodiments of thepresent invention. As shown, the diffusion barrier 510 is disposedbetween a Cu stabilizing core 520 of the wire 500 and an outer bronzematrix 530 containing Nb-based filaments 540. FIG. 6 is across-sectional view of another superconducting wire 600 incorporating adiffusion barrier in accordance with embodiments of the presentinvention. As shown, the diffusion barrier 610 is disposed between innerSb—Cu—Nb-based filaments 620 at the core of the wire 600 and an outer Custabilizer 630.

EXAMPLES

A series of experiments were performed to evaluate Nb—W alloy materialsin terms of processability and thus suitability for use as diffusionbarriers in heavily drawn superconducting wires. The fabrication of thematerials began with the melting of three different Nb—W alloys in abutton hearth. The three different samples had 2.9 weight percent W, 5.7weight percent W, and 11.4 weight percent W, and all three buttonsweighed 680.4 grams after fabrication. Center sections were extractedfrom each of the buttons via cutting on a bandsaw and deburring with afile. The thickness of each section was measured and utilized as thestarting thickness for a series of rolling experiments. The samples wererolled on a mini-mill at a nominal 5% pass schedule. Periodically duringthe rolling, the thicknesses of the samples were measured, and a portionof each sample was extracted for hardness testing. No intermediateannealing or other treatments were performed on the samples. The resultsof the rolling experiments are shown in Table 1 below, which reportsthicknesses and corresponding reductions in area (ROA).

TABLE 1 Reduction in Thickness for Rolling Experiments Sample 1 Nb-2.9%W Sample 2 Nb-5.7% W Sample 3 Nb-11.4% W Thickness ROA Thickness ROAThickness ROA (in) (%) (in) (%) (in) (%) 0.462 0 0.475 0 0.475 0 0.422 90.422 11 0.425 11 0.388 16 0.380 18 0.392 17 0.352 24 0.355 25 0.360 240.290 37 0.292 39 0.292 39 0.255 45 0.255 46 0.260 45 0.190 59 0.192 600.190 60 0.125 73 0.122 74 0.128 73

Subsequently, the hardness of the rolled samples was evaluated using aVickers hardness test using a Vickers test force (HV) of 0.5 kg on a 401MVD Knoop/Vickers Microindentation Tester, available from Wilson WolpertInstruments of Aachen, Germany. Each sample was polished and mountedprior to the hardness testing. Three measurements were performed on eachsample using a 136° pyramidal diamond indenter in accordance with theASTM E384 standard (ASTM International, West Conshohocken, Pa., theentire disclosure of which is incorporated by reference herein), and theaverage and standard deviation were calculated. The results of thehardness tests are reported in Tables 2-4 below.

TABLE 2 Hardness Measurements for Sample 1 (Nb-2.9% W) Hardness (HV0.5)Thickness (in) ROA (%) Test Std. Dev. Average 0.462 0 136.5 1.1 136.1134.9 136.9 0.422 9 125.9 5.2 125.1 129.8 119.5 0.388 16 178.0 6.8 182.4178.9 190.2 0.352 24 183.1 7.1 176.6 169.1 177.1 0.290 37 143.8 5.7142.3 147.1 136.0 0.255 45 184.6 3.1 188.0 190.5 189.0 0.190 59 190.81.8 192.1 191.4 194.2 0.125 73 202.3 5.4 196.1 192.3 193.8

TABLE 3 Hardness Measurements for Sample 2 (Nb-5.7% W) Hardness (HV0.5)Thickness (in) ROA (%) Test Std. Dev. Average 0.475 0 139.3 4.2 141.8139.5 146.7 0.422 11 182.3 3.2 179.2 176.0 179.3 0.390 18 194.2 5.8187.5 184.1 184.2 0.355 25 204.7 7.8 200.5 205.4 191.5 0.292 39 197.910.1 208.9 211.1 217.8 0.255 46 191.4 7.9 199.3 199.2 207.2 0.192 60223.6 10.4 214.3 216.3 203.1 0.122 74 231.3 2.2 228.9 226.9 228.6

TABLE 4 Hardness Measurements for Sample 3 (Nb-11.4% W) Hardness (HV0.5)Thickness (in) ROA (%) Test Std. Dev. Average 0.475 0 188.6 1.0 188.1188.8 186.9 0.425 11 202.1 13.2 216.8 220.9 227.5 0.392 17 220.4 7.0225.5 222.7 233.5 0.360 24 240.3 4.9 245.9 248.6 248.8 0.292 39 236.02.1 234.8 235.9 232.4 0.260 45 260.7 4.0 263.0 260.7 267.6 0.190 60251.2 2.9 254.0 253.9 257.0 0.128 73 256.4 9.5 258.2 249.7 268.4

As indicated on the above data tables, all three of the tested samplesexhibited good ductility when processed via cold working to ROA valuesof over 70%. The measured behavior is similar to that exhibited by pureniobium samples and indicates the suitability of these sample alloys foruse as diffusion barriers in state-of-the-art superconducting wires. Thehardness of each alloy increased slightly as a function of increased Wcontent and increased ROA, as expected, but the samples all exhibitedgood ductility at all conditions tested. None of the samples cracked orwere otherwise damaged by the cold working utilized in the testingprocedure, and all samples deformed quite uniformly during testing.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. A superconducting wire possessing diffusionresistance and mechanical strength, the superconducting wire comprising:an outer wire matrix comprising Cu; disposed within the wire matrix, adiffusion barrier comprising a Nb alloy containing 0.1%-20% W or a Nb—Taalloy containing 0.1%-20% W; and a plurality of composite filamentssurrounded by the diffusion barrier and separated from the outer wirematrix by the diffusion barrier, wherein: each composite filamentcomprises (i) a plurality of monofilaments and (ii) a claddingcomprising Cu surrounding the plurality of monofilaments, eachmonofilament comprises a core comprising Nb and, surrounding the core, acladding comprising Cu, the diffusion barrier occupies 1%-20% of across-sectional area of the superconducting wire, and the diffusionbarrier extends through an axial dimension of the superconducting wire.2. The wire of claim 1, further comprising, disposed between thecomposite filaments and the diffusion barrier, an annular regioncomprising a Nb-based superconducting phase.
 3. The wire of claim 2,wherein the annular region comprises Nb₃Sn.
 4. The wire of claim 2,wherein the annular region conforms to and is contact with the diffusionbarrier.
 5. The wire of claim 1, wherein the diffusion barrier occupies1%40% of the cross-sectional area of the superconducting wire.
 6. Thewire of claim 1, wherein the diffusion barrier occupies 2%40% of thecross-sectional area of the superconducting wire.
 7. The wire of claim1, wherein the diffusion barrier occupies 3%40% of the cross-sectionalarea of the superconducting wire.
 8. The wire of claim 1, wherein thecore of each monofilament comprises Nb alloyed with at least one of Ti,Zr, Hf, Ta, Y, or La.
 9. The wire of claim 1, wherein the core of eachmonofilament comprises Nb₃Sn.
 10. The wire of claim 1, wherein thediffusion barrier comprises Nb-6W or Nb—Ta-3W.
 11. The wire of claim 1,wherein the diffusion barrier additionally contains one or more alloyingelements selected from the group consisting of Ru, Pt, Pd, Rh, Os, Ir,Mo, Re, or Si.
 12. The wire of claim 1, wherein each of the compositefilaments has a hexagonal cross-sectional shape.
 13. The wire of claim1, wherein each of the monofilaments has a hexagonal cross-sectionalshape.
 14. The wire of claim 1, further comprising a stabilizing elementdisposed within the plurality of composite filaments and surrounded bythe diffusion barrier, the stabilizing element comprising a Ta alloycontaining 0.1%-20% W, a Nb alloy containing 0.1%-20% W, or a Nb—Taalloy containing 0.1%-20% W.
 15. The wire of claim 1, wherein a yieldstrength of the superconducting wire is at least 100 MPa. 16.-44.(canceled)